A line code is needed to transmit digital information on physical media from a transmitter to a receiver. Particular physical states of the line can therefore be assigned to binary data. In the case of electrical lines, two different electrical voltages, for example, are used to code a logical “0” and a logical “1” in a serial data stream. The task of the line code is to adapt the data stream to be transmitted to the transmission medium.
During Manchester coding, the logical data “0” and “1” are not coded by an individual voltage level on the electrical line but rather by a voltage edge, that is to say a signal change. In one embodiment of Manchester coding, a logical “1” corresponds to a positive signal change and a logical “0” corresponds to a negative signal change. Correction edges are inserted in the case of successively identical data. The Manchester code thus requires a sequence of “0” and “1” values to also be detected by changing edges.
An advantage of Manchester coding over other line codes is that the clock on which the data are based is implicitly concomitantly transmitted with the transmission of the binary data and the data can be transmitted largely without a DC component. A simple possible way of generating a Manchester code is to use XOR operations between the clock signal present at the transmitter end and the binary data to be transmitted.
At the receiver end, the clock must be determined from the coded signal so that the reception signal can be sampled in the correct phase and the correction edges can be eliminated. In the case of Manchester-coded signals, a series of signal edges must generally be detected for this purpose. The actual transmission of useful data can be started only after the clock has been determined and a clock generation unit at the reception end has been synchronized.
The European patent EP0266085B1 discloses a method in which a special bit string (header) known to the receiver is transmitted. This bit string is then used to synchronize the receiver.
The object of the present invention is to provide a method and an apparatus, with which it becomes possible to decode Manchester-coded signals without using a predetermined, special bit string for synchronization.